Alzheimer's disease is characterized by the accumulation of a 39-43 amino acid peptide termed the β-amyloid protein or Aβ, in a fibrillar form, existing as aggregates in extracellular amyloid plaques and as amyloid within the walls of cerebral blood vessels. Fibrillar Aβ amyloid aggregate deposition in Alzheimer's disease is believed to be detrimental to the patient and eventually leads to toxicity and neuronal cell death, characteristic hallmarks of Alzheimer's disease. Accumulating evidence implicates amyloid, and more specifically, the formation, deposition, accumulation and/or persistence of Aβ aggregatess, as a major causative factor of Alzheimer's disease pathogenesis. In addition, besides Alzheimer's disease, a number of other amyloid diseases involve formation, deposition, accumulation and persistence of Aβ aggregatess, including Down's syndrome, disorders involving congophilic angiopathy, such as but not limited to, hereditary cerebral hemorrhage of the Dutch type, and cerebral β-amyloid angiopathy.
Parkinson's disease is another human disorder characterized by the formation, deposition, accumulation, aggregation and/or persistence of abnormal fibrillar protein deposits that demonstrate many of the characteristics of amyloid. In Parkinson's disease, an accumulation of cytoplasmic Lewy bodies consisting of aggregates of filaments of α-synuclein are believed important in the pathogenesis and as therapeutic targets. New agents or compounds able to inhibit α-synuclein formation, deposition, accumulation, aggregation and/or persistence, or disrupt pre-formed α-synuclein fibrils or aggregates (or portions thereof) are regarded as potential therapeutics for the treatment of Parkinson's and related synucleinopathies. A 35 amino acid fragment of α-synuclein that has the ability to form amyloid-like fibrils or aggregates either in vitro or as observed in the brains of patients with Parkinson's disease. The fragment of α-synuclein is a relative important therapeutic target as this portion of α-synuclein is believed crucial for formation of Lewy bodies as observed in all patients with Parkinson's disease, synucleinopathies and related disorders. In addition, the α-synuclein protein which forms fibrils or aggregates, and is Congo red and Thioflavin S positive (specific stains used to detect amyloid fibrillar aggregates), is found as part of Lewy bodies in the brains of patients with Parkinson's disease, Lewy body disease (Lewy in Handbuch der Neurologie, M. Lewandowski, ed., Springer, Berlin pp. 920-933, 1912; Pollanen et al, J. Neuropath. Exp. Neurol. 52:183-191, 1993; Spillantini et al, Proc. Natl. Acad. Sci. USA 95:6469-6473, 1998; Arai et al, Neurosci. Lett. 259:83-86, 1999), multiple system atrophy (Wakabayashi et al, Acta Neuropath. 96:445-452, 1998), dementia with Lewy bodies, and the Lewy body variant of Alzheimer's disease. In Parkinson's disease, aggregates develop in the brains of patients with this disease which are Congo red and Thioflavin S positive, and which contain predominant beta-pleated sheet secondary structure.
Amyloid as a Therapeutic Target for Alzheimer's Disease
Alzheimer's disease also puts a heavy economic burden on society. A recent study estimated that the cost of caring for one Alzheimer's disease patient with severe cognitive impairments at home or in a nursing home, is more than $47,000 per year (A Guide to Understanding Alzheimer's Disease and Related Disorders). For a disease that can span from 2 to 20 years, the overall cost of Alzheimer's disease to families and to society is staggering. The annual economic toll of Alzheimer's disease in the United States in terms of health care expenses and lost wages of both patients and their caregivers is estimated at $80 to $100 billion (2003 Progress Report on Alzheimer's Disease).
Tacrine hydrochloride (“Cognex”), the first FDA approved drug for Alzheimer's disease, is a acetylcholinesterase inhibitor (Cutler and Sramek, N. Engl. J. Med. 328:808 810, 1993). However, this drug has showed limited success in producing cognitive improvement in Alzheimer's disease patients and initially had major side effects such as liver toxicity. The second FDA approved drug, donepezil (“Aricept”), which is also an acetylcholinesterase inhibitor, is more effective than tacrine, by demonstrating slight cognitive improvement in Alzheimer's disease patients (Barner and Gray, Ann. Pharmacotherapy 32:70-77, 1998; Rogers and Friedhoff, Eur. Neuropsych. 8:67-75, 1998), but is not believed to be a cure. Therefore, it is clear that there is a need for more effective treatments for Alzheimer's disease patients.
Alzheimer's disease is characterized by the deposition and accumulation of a 39-43 amino acid peptide termed the beta-amyloid protein, Aβ or β/A4 (Glenner and Wong, Biochem. Biophys. Res. Comm. 120:885-890, 1984; Masters et al., Proc. Natl. Acad. Sci. USA 82:4245-4249, 1985; Husby et al., Bull. WHO 71:105-108, 1993). Aβ is derived by protease cleavage from larger precursor proteins termed β-amyloid precursor proteins (APPs) of which there are several alternatively spliced variants. The most abundant forms of the APPs include proteins consisting of 695, 751 and 770 amino acids (Tanzi et al., Nature 31:528-530, 1988).
The small Aβ peptide is a major component that makes up the amyloid deposits of “plaques” in the brains of patients with Alzheimer's disease. In addition, Alzheimer's disease is characterized by the presence of numerous neurofibrillary “tangles”, consisting of paired helical filaments which abnormally accumulate in the neuronal cytoplasm (Grundke-Iqbal et al., Proc. Natl. Acad. Sci. USA 83:4913-4917, 1986; Kosik et al., Proc. Natl. Acad. Sci. USA 83:4044-4048, 1986; Lee et al., Science 251:675-678, 1991). The pathological hallmark of Alzheimer's disease is therefore the presence of “plaques” and “tangles”, with β-amyloid being deposited in the central core of the plaques. The other major type of lesion found in the Alzheimer's disease brain is the accumulation of β-amyloid in the walls of blood vessels, both within the brain parenchyma and in the walls of meningeal vessels that lie outside the brain. The β-amyloid deposits localized to the walls of blood vessels are referred to as cerebrovascular amyloid or congophilic angiopathy (Mandybur, J. Neuropath. Exp. Neurol. 45:79-90, 1986; Pardridge et al., J. Neurochem. 49:1394-1401, 1987).
For many years there has been an ongoing scientific debate as to the importance of “β-amyloid” in Alzheimer's disease, and whether the “plaques” and “tangles” characteristic of this disease were a cause or merely a consequence of the disease. Within the last few years, studies now indicate that β-amyloid is indeed a causative factor for Alzheimer's disease and should not be regarded as merely an innocent bystander. The Alzheimer's Aβ protein in cell culture has been shown to cause degeneration of nerve cells within short periods of time (Pike et al., Br. Res. 563:311-314, 1991; J. Neurochem. 64:253-265, 1995). Studies suggest that it is the fibrillar structure (consisting of a predominant β-pleated sheet secondary structure), which is responsible for the neurotoxic effects. Aβ has also been found to be neurotoxic in slice cultures of hippocampus (Harrigan et al., Neurobiol. Aging 16:779-789, 1995) and induces nerve cell death in transgenic mice (Games et al., Nature 373:523-527, 1995; Hsiao et al., Science 274:99-102, 1996). Injection of the Alzheimer's Aβ into rat brain also causes memory impairment and neuronal dysfunction (Flood et al., Proc. Natl. Acad. Sci. USA 88:3363-3366, 1991; Br. Res. 663:271-276, 1994).
Probably, the most convincing evidence that Aβ amyloid is directly involved in the pathogenesis of Alzheimer's disease comes from genetic studies. It was discovered that the production of Aβ can result from mutations in the gene encoding, its precursor, β-amyloid precursor protein (Van Broeckhoven et al., Science 248:1120-1122, 1990; Murrell et al., Science 254:97-99, 1991; Haass et al., Nature Med. 1:1291-1296, 1995). The identification of mutations in the beta-amyloid precursor protein gene that cause early onset familial Alzheimer's disease is the strongest argument that amyloid is central to the pathogenetic process underlying this disease. Four reported disease-causing mutations have been discovered which demonstrate the importance of Aβ in causing familial Alzheimer's disease (reviewed in Hardy, Nature Genet. 1:233-234, 1992). All of these studies suggest that providing a drug to reduce, eliminate or prevent fibrillar Aβ formation, aggregation, deposition, accumulation and/or persistence in the brains of human patients will serve as an effective therapeutic.
Parkinson's Disease and Synucleinopathies
Parkinson's disease is a neurodegenerative disorder that is pathologically characterized by the presence of intracytoplasmic Lewy bodies (Lewy in Handbuch der Neurologie, M. Lewandowski, ed., Springer, Berlin, pp. 920-933, 1912; Pollanen et al., J. Neuropath. Exp. Neurol. 52:183-191, 1993), the major components of which are filaments consisting of α-synuclein (Spillantini et al., Proc. Natl. Acad. Sci. USA 95:6469-6473, 1998; Arai et al., Neurosci. Lett. 259:83-86, 1999), an 140-amino acid protein (Ueda et al., Proc. Natl. Acad. Sci. USA 90:11282-11286, 1993). Two dominant mutations in α-synuclein causing familial early onset Parkinson's disease have been described suggesting that Lewy bodies contribute mechanistically to the degeneration of neurons in Parkinson's disease and related disorders (Polymeropoulos et al., Science 276:2045-2047, 1997; Kruger et al., Nature Genet. 18:106-108, 1998). Recently, in vitro studies have demonstrated that recombinant α-synuclein can indeed form Lewy body-like fibrils or aggregates (Conway et al., Nature Med. 4:1318-1320, 1998; Hashimoto et al., Brain Res. 799:301-306, 1998; Nahri et al., J. Biol. Chem. 274:9843-9846, 1999). Most importantly, both Parkinson's disease-linked α-synuclein mutations accelerate this aggregation process, demonstrating that such in vitro studies may have relevance for Parkinson's disease pathogenesis. Alpha-synuclein aggregation and fibril formation fulfills the criteria of a nucleation-dependent polymerization process (Wood et al., J. Biol. Chem. 274:19509-19512, 1999). In this regard α-synuclein fibril formation or aggregation resembles that of Alzheimer's β-amyloid protein (Aβ) fibrils. Alpha-synuclein recombinant protein, and non-Aβ component (known as NAC), which is a 35-amino acid peptide fragment of α-synuclein, both have the ability to form fibrils or aggregate when incubated at 37° C., and are positive with amyloid stains such as Congo red (demonstrating a red/green birefringence when viewed under polarized light) and Thioflavin S (demonstrating positive fluorescence) (Hashimoto et al., Brain Res. 799:301-306, 1998; Ueda et al., Proc. Natl. Acad. Sci. USA 90:11282-11286, 1993).
Synucleins are a family of small, presynaptic neuronal proteins composed of α-, β-, and γ-synucleins, of which only α-synuclein aggregates have been associated with several neurological diseases (Ian et al., Clinical Neurosc. Res. 1:445-455, 2001; Trojanowski and Lee, Neurotoxicology 23:457-460, 2002). The role of synucleins (and in particular, alpha-synuclein) in the etiology of a number of neurodegenerative diseases has developed from several observations. Pathologically, synuclein was identified as a major component of Lewy bodies, the hallmark inclusions of Parkinson's disease, and a fragment thereof was isolated from amyloid plaques of a different neurological disease, Alzheimer's disease. Biochemically, recombinant α-synuclein was shown to form fibrils or aggregates that recapitulate the ultrastructural features of alpha-synuclein isolated from patients with dementia with Lewy bodies, Parkinson's disease and multiple system atrophy. Additionally, the identification of mutations within the synuclein gene, albeit in rare cases of familial Parkinson's disease, demonstrated an unequivocal link between synuclein pathology and neurodegenerative diseases. The common involvement of α-synuclein in a spectrum of diseases such as Parkinson's disease, dementia with Lewy bodies, multiple system atrophy and the Lewy body variant of Alzheimer's disease has led to the classification of these diseases under the umbrella term of “synucleinopathies”.
Parkinson's disease α-synuclein fibrils or aggregates, and the Aβ fibrils of Alzheimer's disease, both consist of a predominantly β-pleated sheet structure. Compounds found to inhibit Alzheimer's disease Aβ amyloid fibril formation have also been shown to be effective in the inhibition of α-synuclein fibril formation or aggregation, as illustrated in the Examples of the present invention. These compounds would therefore also serve as therapeutics for Parkinson's disease and other synucleinopathies, in addition to having efficacy as a therapeutic for Alzheimer's disease.
Parkinson's disease and Alzheimer's disease are characterized by the inappropriate accumulation of insoluble aggregates comprised primarily of misfolded proteins that are enriched in β-pleated sheet secondary structure (reviewed in Cohen et al., Nature 426:905-909, 2003; Chiti et al., Annu. Rev. Biochem., 75:333-366, 2006). In Parkinson's disease, α-synuclein is the major constituent of these aggregates, as part of Lewy Bodies, and mutations in α-synuclein that increase its propensity to misfold and aggregate are observed in familial Parkinson's disease (Polymeropoulos et al., Science 276:1197-1199, 1997; Papadimitriou et al., Neurology 52:651-654, 1999).
Mitochondrial dysfunction, specifically as a result of impairment at complex I of the electron transport chain, is also a common feature of Parkinson's disease (Schapira et al., J. Neurochem., 54:823-827, 1990; reviewed in Greenamyre et al., IUBMB Life, 52:135-141, 2001). Direct evidence for mitochondrial deficits in the etiology of Parkinson's disease came first from the observation that MPP+ (1-methyl-4-phenyl-2,3-dihydropyridinium), the active metabolite of the parkinsonism toxin N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), inhibits complex I (Nicklas et al., Life Sci., 36:2503-2508, 1985). Subsequently, rotenone, another complex I inhibitor, was shown to be an improved model for α-synuclein aggregation because it reproduces the above-mentioned α-synuclein-positive intracytoplasmic aggregates, in addition to the behavioral changes and loss of dopaminergic neurons seen in the MPTP model. Rotenone toxicity of this type is seen in multiple model systems including rats (Betarbet et al., Nat. Neurosci., 3:1301-1306, 2000; Panov et al., J. Biol. Chem., 280:42026-42035, 2005), rat brain slices (Sherer et al., J. Neurosci., 23:10756-10764, 2003; Testa et al., Mol. Brain Res., 134:109-118, 2005), C. elegans (Ved et al., J. Biol. Chem., 280:42655-42668, 2005) and cultured cells (Sherer et al., J. Neurosci., 22:7006-7015, 2002) and has been shown to be a consequence of increased oxidative damage resulting from complex I inhibition.
To better understand the relationship of oxidative damage to mutant α-synuclein pathogenesis, a neuroblastoma cell line (using BE-M17 cells) has been established in the art that overexpresses A53T α-synuclein. In these cells, A53T α-synuclein aggregates in response to a variety of oxidative stress-inducing agents and potentiates mitochondrial dysfunction and cell death (Ostrerova-Golts et al., J. Neurosci., 20:6048-6054, 2000). These cells are amenable to rotenone treatment as an oxidative stress inducer and hence, are particularly useful for testing agents that might inhibit α-synuclein aggregation/fibrillogenesis.
Discovery and identification of new compounds or agents as potential therapeutics to arrest amyloid formation, deposition, accumulation and/or persistence that occurs in Alzheimer's disease, and Parkinson's disease, are desperately sought.